(1) Field of the Invention
The present invention relates to a structure of mounting a semiconductor element onto a substrate and a mounting method thereof, and in particular relates to a structure of mounting a semiconductor element onto a substrate and a mounting method thereof wherein a semiconductor element (chip) having bumps is mounted to a conductive portion of a flexible substrate or the like.
(2) Description of the Prior Art
Referring to FIGS. 1A-1D, the prior art will be described. FIGS. 1A through 1D show procedural diagrams showing a mounting method of a semiconductor element of a conventional example.
Consider a case as shown in FIG. 1A where a semiconductor chip 103 having bumps 102 consisting of gold is mounted to a flexible substrate 101 having a conductor pattern 100 as the conductive portion. Here, the surface of conductor pattern 100 is plated with tin. As to the number of the bumps, which corresponds to the number of terminals if the semiconductor chip is used as an IC, the chip has tens to hundreds of bumps, in general.
First, as shown in Fig. 1B, the substrate and the chip are placed so that bumps 102 contact conductor pattern 100, and then they are heated and pressed to each other. This process is performed at a temperature of 280.degree. C. to 600.degree. C., and each bump is about 100 .mu.m square and is pressed with a load of 10 to 60 gf. As a result, bumps 102 and the tin plated on the surface of the conductor pattern, form an alloy layer 104 consisting of gold and tin, whereby semiconductor chip 103 is fixed and electrically connected to flexible substrate 101.
Next, as shown in FIG. 1C, fluid resin 105 is applied between semiconductor chip 103 and flexible substrate 101 and also fills up the chip side, finally producing a mounted structure of the semiconductor chip as shown in FIG. 1D.
FIG. 2 is a sectional view showing a mounting structure of a semiconductor element in accordance with another conventional example. Components having the same functions are allotted with the same reference numerals as those in FIGS. 1A-1D. In this example, an anisotropic conductive film 107 with conductive particles 106 dispersed therein is applied onto the surface of a flexible substrate 101 with a conductor pattern 100 formed thereon. A semiconductor chip 103 having bumps 102 is pressed over this anisotropic conductive film 107 while being heated. Bumps 102 and conductor pattern 100 on flexible substrate 101 are electrically connected by conductive particles 106 within anisotropic conductive film 107. Semiconductor chip 103 and flexible substrate 101 are bonded by anisotropic conductive film 107 cured by heat.
The applied pressure in this case is similar to that in the case of FIGS. 1A-1D while the temperature causing formation of an alloy layer as in FIGS. 1A-1D is not needed but it is only necessary to cure anisotropic conductive film 107; this means that the temperature is set at about 200.degree. C.
In the conventional example shown in FIGS. 1A-1D, electrical tests, etc., are implemented for the finished product as shown in FIG. 1D. That is, the product is examined after alloy layer 104 has been formed between bumps 102 and conductor pattern 100 of flexible substrate 101, and resin 105 as filler has been applied. In this case, however, semiconductor chip 103 has already been fixed firmly to flexible substrate 101 by alloy layer 104.
Accordingly, if, from the electrical tests etc., semiconductor chip 103 turns out to be defective after the completion of the product, it is necessary to peel off conductor pattern 100 of flexible substrate 101 in order to remove the defective semiconductor chip 103. This means that flexible substrate 101 can not be reused, resulting in waste.
Further, in the heating step shown in FIG. 1B where alloy layer 104 is formed, tin on the surface of conductor pattern 100 of flexible substrate 101 tends to gather toward bumps 102. As a result, there are cases where alloy layer 104 largely spreads further out from the joint area between a bump 102 of semiconductor chip 103 and conductor pattern 100. The thus formed alloy layer 104 directly comes in contact with other neighboring bumps or other conductor patterns, causing edge leakage.
Further, since this substrate is a flexible one, if substrate 101 becomes bent at a portion A encircled in FIG. 3, the overrun alloy layer 104 is liable to contact the end face of semiconductor chip 103.
In the mounting structure and mounting method shown in FIG. 2, anisotropic conductive film 107 was used. However, conductive particles 106 can not always be dispersed uniformly within the film, conductive particles 106 may exist in relatively large densities at some places. These sites with a high particle density could cause damage to a circuit to be a joint area to semiconductor chip 103.
Further, conductive particles 106 should ideally function as electrical communication between bumps 102 and flexible substrate 101, but ununiformity of conductive particles 106 heightens the connection resistance, or causes unreliable connection, in the worst case, causing disconnection.